1. Field of the Invention
The present invention relates to a plasma arc power supply of a welding machine or a cutting machine that uses a plurality of power supply units, and to a control method for the plasma arc power supply.
2. Description of the Related Art
An inverter plasma arc power supply is well known conventionally as a plasma arc power supply used in welding machines or cutting machines. The capacity of welding machines or cutting machines is adjusted from a small-current region to a large-current region for use according to the application.
Japanese Laid-open Patent Application No. 8-1350 (Parallel System of Inverter Resistance Welding Machine) discloses a technique connecting each of multiple inverter resistance welding machines having the same configuration via a welding transformer for each inverter, forming a direct parallel connection to the secondary side of the welding transformer, and enabling increased capacity in a power supply through the use of a synchronization signal and a communication means. A plurality of power supply units composed of inverters having a standard capacity is connected in parallel to form a power supply apparatus, and the inverters are activated in synchrony by a synchronization signal created from the zero crossing of an alternating-current power supply. The capacity can therefore be easily increased and reduced, and replacement can be performed in a short time in the event of failure.
The energy P of welding or cutting can be calculated as the product of the inductance value L and the square of the current I (P=(½)×L×I×I). When the power supply units are activated in synchrony as described in Japanese Laid-open Patent Application No. 8-1350, the output of the current is distributed among the power supply units, and the energy P when the number of power supply units is 4, for example, is P=4×{(½)×L×(I/4)×(I/4)}. Accordingly, the overall P=(⅛)L×I×I, and the energy P is one-fourth the energy obtained when a single power source is used. Such a loss in energy P creates poor efficiency in a small-current region in which only a low energy is originally needed. When four power supply units are used, the current of each power supply unit must be multiplied by four, or a complex calculation must be performed to vary the inductance L in each case in order to obtain the same energy obtained when a single power supply is used. In the invention disclosed in Japanese Laid-open Patent Application No. 8-1350, since all of the power supply units are driven at the same time, an unstable state at the time of plasma arc generation, as well as wandering of the current, are added to the plurality of power supply units. Drawbacks therefore occur in that the outputted load current becomes more unstable, and the load current takes time to be stabilized.
The present inventors therefore developed the invention described in Japanese Laid-open Patent Application No. 6-190562 (Power Supply for Plasma Arc) prior to Japanese Laid-open Patent Application No. 8-1350. Japanese Laid-open Patent Application No. 6-190562 discloses a power supply provided with N direct-current power supply devices in which the direct-current power supply devices are operated in parallel after only a prescribed direct-current power supply device is operated to supply a pilot arc current to a load. The power supply includes an integrator for generating an integrated signal and an inverted signal thereof from a signal from a parallel operation command terminal to which a parallel operation command signal is fed, a first multiplier for multiplying a reference signal and an inverted signal of a charge fed to the load, an attenuator for attenuating the reference signal to 1/N, a second multiplier for multiplying the integrated signal by the output signal of the attenuator, and an addition unit for adding the output signals of the first multiplier and the second multiplier, wherein the output setting signals of the remaining direct-current power supply devices are formed by a prescribed direct-current power supply device according to the output signal of the addition unit. Through this configuration, a prescribed direct-current power supply device is first operated independently to generate a pilot arc, the prescribed direct-current power supply device is then smoothly reduced to a current value that corresponds to 1/N of the reference power supply, and the other direct-current power supply devices can be smoothly increased to the current value that corresponds to 1/N of the reference power supply and promptly transferred to parallel operation, whereby the power feed during arc startup is stabilized. According to the invention disclosed in Japanese Laid-open Patent Application No. 6-190562, all of the operational effects disclosed in Japanese Laid-open Patent Application No. 8-1350 can be achieved, and stability during arc startup can also be obtained as an effect.
The control timing of the current of the direct-current power supply device according to Japanese Laid-open Patent Application No. 6-190562 will be described using FIG. 5. In FIG. 5, N is 4, the prescribed direct-current power supply device is the power supply unit U1, and the (N−1) direct-current power supply devices are the three power supply units U2, U3, U4. When IC is obtained as the load current, a pilot arc current IP is conducted to the electrode by the power supply unit U1 at t1 to initiate the pilot arc, the torch is brought near the workpiece to generate a plasma arc current Ia. The plasma arc current Ia is detected, whereby the power supply unit U1 is shift to the main arc state, and the power supply units U2, U3, U4 are driven at the same time. The arc current of the power supply unit U1 is controlled so as to decrease gradually to Ia/4, and the arc currents of each of the power supply units U2, U3, U4 are controlled so as to gradually increase to Ia/4. A stable load current Ia is obtained by adding all of the currents obtained in this manner, so that welding or cutting is performed. When the power supply is then adjusted to change the load current to Ib, the arc currents of all the power supply units U1 through U4 can be increased to Ib/4 and added to obtain the load current Ib.
However, the invention disclosed in Japanese Laid-open Patent Application No. 6-190562 also has drawbacks in that parallel driving brings the energy to 1/N, particularly in the small-current region. When an extremely thin metal plate is welded or cut as the workpiece, or when only a small current is needed, such as when, for example, etching letters or symbols using the low current region on a workpiece, all of the direct-current power supply devices are used, and work therefore cannot be performed at a power savings, and energy is wasted.
When, for example, a current of 100 A is needed as the plasma arc current Ia, the current handled by the four power supply units is 25 A each, and all of the power supply units are driven in order to obtain a current similar to the pilot current. When all of the power supply units abruptly increase and decrease due to the effects of noise or the like in such a small-current region, the fluctuations of all the power supply units are added together, and the effects of noise and the like are increased by a factor of four. The effect is small when a large current is fed, but the temperature of the torch rapidly increases when there is significant fluctuation in the current value in the small-current region, and there is a risk of electrode deformation or interruption of the arc. When the inductance is L, the energy P is 5000×L in one power supply unit, whereas the energy P is reduced to 1250×L by the driving of four power supply units. It has been suggested that this drawback could be overcome by varying the inductance L in order to obtain a certain degree of energy according to the application, but there is no longer the merit of using multiple power supply units whereby the variable range of the inductance is reduced, a wide range of load currents is obtained using a simple structure, and failure response and equipment outlay are facilitated. Considering the environment, there is also a societal need to obtain energy savings with minimal waste of power using a simple configuration in the small-current region in which power consumption is considered to be naturally low.
Therefore, an object of the present invention is to reduce energy waste in the small-current region and apply a stable plasma arc power supply by controlling the drive timing of a plurality of power supply units.